Encryption apparatus and method in a wireless communications system

ABSTRACT

An encryption apparatus and method for generating a ciphertext from an input plaintext of the same length as the ciphertext by parallel processing of the input signal. Since a non-delayed signal is synchronized to a delayed signal, an accurate ciphertext is produced. Therefore, the encryption speed is increased, the number of devices for timing synchronization is reduced, an encryption system is stabilized, and production cost is reduced.

PRIORITY

This application claims priority under 35 U.S.C. § 119 to an application entitled “Encryption Apparatus and Method in a Wireless Communications System” filed in the Korean Industrial Property Office on Oct. 8, 2002 and assigned Serial No. 2002-61179, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a wireless communications system, and in particular, to an encryption apparatus and method for implementing confidentiality and integrity algorithms in a wireless communications system.

2. Description of the Related Art

As the first generation analog encryption system has evolved into the second generation digital encryption system, more advanced encryption techniques have been used. The current third generation encryption system provides encryption service for multimedia service i.e., audio and video information. Thus, the importance of encryption has increased in order to provide confidentiality to voice signals, multimedia service, and user data. An integrity algorithm is required to authenticate control signals between mobile terminals in a wireless communication system and a network. The 3^(rd) Generation Project Partnership (3GPP) has selected the KASUMI algorithm as the f8 confidentiality and f9 integrity algorithms for a third generation system based on a Global System for Mobile communication (GSM) core network, and a Universal Mobile Telecommunication System (UMTS).

FIG. 1 is a block diagram illustrating an example of a conventional KASUMI algorithm. Referring to FIG. 1, KASUMI is an 8-round Feistel unit cipher that provides a 64-bit output ciphertext from a 64-bit input plaintext with 8-round encryption. The 64-bit input signal is divided into a 32-bit signal L₀ and a 32-bit signal R₀. FLi units (1≦i≦8) 110 to 180 and FOi units (1≦i≦8) 210 to 280 encrypt the signals L₀ and R₀ under corresponding encryption keys KL_(i) (1≦i≦8), KO_(i) (1≦i≦8), and KI_(i) (1≦i≦8) and output the 64-bit ciphertext.

Encryption in accordance with FIG. 1 occurs in the following manner. An FL1 unit 110 encrypts the input 32-bit signal L₀ with an encryption key KL₁ and outputs a ciphertext L₀₁. An FO1 unit 210 encrypts the 32-bit ciphertext L₀₁ with encryption keys KO₁ and KI₁ and outputs a ciphertext L₀₂. An Exclusive-OR operation is performed to logically “exclusive OR” the ciphertext L₀₂ and the 32-bit signal R₀ to provide a 64-bit ciphertext. This encryption occurs eight times and a final 64-bit ciphertext is generated in the KASUMI.

FIG. 2A is a block diagram illustrating an example of FOi units. Referring to FIG. 2A, FOi denotes an ith FO unit. The FOi unit comprises a plurality of f1_(i,j) sub-ciphers (1≦i≦3, 1≦i≦3) to provide 3-rounds of encryption. Here, the operation of the FO1 unit 210 will be described by way of example. The 32-bit input signal is divided into two 16-bit signals L₀ and R₀. An Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal L₀ and a 16-bit sub-encryption key KO_(1,1), to provide a signal L₁. A f1_(1,1) sub-cipher 201 encrypts the signal L₁ with a 16-bit sub-encryption key KI_(1,1) and outputs a signal L_(1D). Meanwhile, a first delay (D₁) 10 delays the 16-bit signal R₀, which is equivalent to the signal R₁, in order to synchronize the 16-bit signal R₀ with the signal L_(1D) and output a delayed signal R_(1D). For a second-round of encryption, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal RID and a 16-bit sub-encryption key KO_(1,2) to provide a signal L₂. A f1_(1,2) sub-cipher 203 encrypts the signal L₂ with a 16-bit sub-encryption key KI_(1,2) and outputs a signal L_(2D). Meanwhile, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R_(1D) and the signal L_(1D), to provide a signal R₂. A second delay (D₂) 20 delays the signal R₂ in order to synchronize the signal R2 with the signal L_(2D) and output a delayed signal R_(2D). For a third-round of encryption, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R_(2D) and a 16-bit sub-encryption key KO_(1,3), resulting in a signal L₃. A f1_(1,3) sub-cipher 205 encrypts the signal L₃ with a 16-bit sub-encryption key KI_(1,3) and outputs a signal L_(3D). Meanwhile, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R_(2D) and the signal L_(2D) to provide a signal R₃. A third delay (D₃) 30 delays the signal R₃ in order to synchronize the signal R₃ with the signal L_(3D) and output a delayed signal R_(3D). An Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R_(3D) and the signal L_(3D), to provide a signal R₄. The 16-bit signal R₄ is operated with the 16-bit signal R_(3D) (=L₄), resulting in a 32-bit ciphertext L₄//R₄.

The FO1 unit uses the three delays 10, 20 and 30 to synchronize to the output timings of the sub-ciphers 201, 203 and 205.

FIG. 2B is a block diagram illustrating another example of the FOi units. Referring to FIG. 2B, a FOi unit comprises a plurality of f1_(i′,j′) sub-ciphers (1≦i′≦3 1≦j′≦3), for 3-rounds of encryption. Here, the FO1 unit 210 will be described by way of example. The 32-bit input signal is divided into two 16-bit signals L_(0′) and R_(0′). An Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal L_(0′) and a 16-bit sub-encryption key KO_(1,1), to provide a signal L_(1′). A f1_(1′,1′) sub-cipher 211 encrypts the signal L_(1′) with the 16-bit sub-encryption key KI_(1,1) and outputs a signal L_(1D′). Meanwhile, a fourth delay (D₄) 40 delays the 16-bit signal R_(0′) (=R_(1′)) and outputs a delayed signal R_(1D′). An Exclusive-OR operation is performed to logically “exclusive OR” the signals L_(1D′) and R_(1D′) to provide a signal L_(2′). Simultaneously, an Exclusive-OR operation is performed to logically “exclusive OR” the 16-bit signal R_(0′) and a 16-bit sub-encryption key KO_(1,2), to provide a signal R_(2′). A f1_(1′,2′) sub-cipher 213 encrypts the signal R_(2′) with a 16-bit sub-encryption key KI_(1,2) and outputs a signal R_(2D′). An Exclusive-OR operation is performed to logically “exclusive OR” the signals L_(2′) and R_(2D′) to provide a signal R_(3′). Another Exclusive-OR operation is performed to logically “exclusive OR” the signal L_(2′) and a 16-bit sub-encryption key KO_(1,3), to provide a signal L_(3′). A f1_(1′,3′) sub-cipher 215 encrypts the signal L_(3′) with a 16-bit sub-encryption key KI_(1,3) and outputs a signal L_(3D′). Meanwhile, a fifth delay (D₅) 50 delays the signal R_(3′) and outputs a delayed signal R_(3D′). An Exclusive-OR operation is performed to logically “exclusive OR” the signals L_(3D′) and R_(3D′) to provide a 16-bit signal L_(4′). The 16-bit signal L_(4′) is operated with the 16-bit signal R_(3D′) (=R_(4′)), resulting in a 32-bit ciphertext L_(4′)//R_(4′).

The above advanced FOi unit uses the two delays 40 and 50 to synchronize to the output timings of the F1 sub-ciphers 211 and 215. However, due to the use of the delays, a large chip capacity is required.

FIG. 3 is a block diagram illustrating an example of the f1_(i,j) sub-ciphers illustrated in FIGS. 2A and 2B. By way of example, the f1_(1,1) sub-cipher 201 will be described below. Referring to FIG. 3, the 16-bit input signal is divided into a 9-bit signal RL₀ and a 7-bit signal RR₀. An SBox91 (S91) operator 310 generates a 9-bit signal y0, y1, . . . , y8 from the input signal RL₀ using y0=(x0x2)⊕x3⊕(x2x5)⊕(x5x6)⊕(x0x7)⊕(x1x7)⊕(x2x7)⊕(x4x8)⊕(x5x3)⊕(x7x8)⊕‘1’; y1=x1⊕(x0x1)⊕(x2x3)⊕(x0x4)⊕(x1x4)⊕(x0x5)⊕(x3x5)⊕x6⊕(x1x7)⊕(x2x7)⊕(x5x8)⊕‘1’; y2=x1⊕(x0x3)⊕(x3x4)⊕(x0x5)⊕(x2x6)⊕(x3x6)⊕(x5x6)⊕(x4x7)⊕(x5x7)⊕(x6x7)⊕x8⊕(x0x8)⊕‘1’; y3=x0⊕(x1x2)⊕(x0x3)⊕(x2x4)⊕x5⊕(x0x6)⊕(x1x6)⊕(x4x7)⊕(x0x8)⊕(x1x8)⊕(x7x8); y4=(x0x1)⊕(x1x3)⊕x4⊕(x0x5)⊕(x3x6)⊕(x0x7)⊕(x6x7)⊕(x1x0)⊕(x2x3)⊕(x3x0); y5=x2⊕(x1x4)⊕(x4x5)⊕(x0x6)⊕(x0x6)⊕(x3x7)⊕(x4x7)⊕(x6x7)⊕(x5x3)⊕(x6x8)⊕(x7x8)⊕‘1’; y6=x0⊕(x2x3)⊕(x1x5)⊕(x2x5)⊕(x4x5)⊕(x3x6)⊕(x4x6)⊕(x5x6)⊕x7⊕(x1x8)⊕(x3x8)⊕(x5x8)⊕(x7x8); y7=(x0x1)⊕(x0x2)⊕(x1x2)⊕x3⊕(x0x3)⊕(x2x3)⊕(x4x5)⊕(x2x6)⊕(x3x5)⊕(x2x7)⊕(x5x7)⊕(x8⊕‘1’; y8=(x0x1)⊕x2⊕(x1x2)⊕(x3x4)⊕(x1x5)⊕(x2x5)⊕(x1x6)⊕(x4x6)⊕x7⊕(x2x8)⊕(x3x8);   (1)

A ZE1 unit 320 receives the signal RR₀, adds two zeroes to the Most Significant Bit (MSB) of the signal RR₀, and outputs a 9-bit signal. An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the S91 operator 310 and the ZE1 unit 320 to provide a 9-bit signal RL₁. Another Exclusive-OR operation is performed to logically “exclusive OR” the signal RL₁ and a 9-bit sub-encryption key KI_(1,1,2), to provide a 9-bit signal RL₂.

A TR1 unit 330 removes two zero bits from the MSBs of the 9-bit signal RL₁. An SBox71 (S71) operator 340 generates a 7-bit signal y0, y1, . . . , y6 from the input signal RR₀ (=RR₁) by y0=(x1x3)⊕x4⊕(x0x1x1)⊕x5⊕(x2x5)⊕(x3x4x5)⊕x6⊕(x0x6)⊕(x1x6)⊕(x3x6)⊕(x2x4x6)⊕(x1x5x6)⊕(x4x5x6); y1=(x0x1)⊕(x1x4)⊕(x2x4)⊕x5⊕(x1x2x5)⊕(x0x3x5)⊕x5⊕(x0x2x3)⊕(x3x6)⊕(x4x5x6)‘1’; y2=x0⊕(x0x3)⊕(x2x3)⊕(x1x2x4)⊕(x0x3x4)⊕(x1x5)⊕(x0x2x5)⊕(x0x5)⊕(x0x1x6)⊕(x2x6)⊕(x1x6)⊕‘1’; y3=x1⊕(x0x1x2)⊕(x1x4)⊕(x3x4)⊕(x0x5)⊕(x0x1x5)⊕(x2x3x5)⊕(x1x4x5)⊕(x2x6)⊕(x1x3x6); y4=(x0x2)⊕x3⊕(x1x3)⊕(x1x4)⊕(x0x1x4)⊕(x2x3x4)⊕(x0x5)⊕(x1x3x5)⊕(x0x4x5)⊕(x1x6)⊕(x3x6)⊕(x0x3x6)⊕(x5x6)⊕‘1’; y5=x2⊕(x0x2)⊕(x0x3)⊕(x1x2x3)⊕(x0x1x4)⊕(x0x5)⊕(x2x5)⊕(x4x5)⊕(x1x6)⊕(x1x2x6)⊕(x0x3x6)⊕(x3x4x6)⊕(x2x5x6)⊕‘1’; y6=(x1x2)⊕(x1x1x3)⊕(x0x4)⊕(x1x5)⊕(x3x5)⊕x6⊕(x0x1x6)⊕(x2x3x6)⊕(x1x4x5)⊕(x0x5x6);  (2)

An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the TR₁ 330 and the S71 operator 340 via a sub-encryption key KI_(1,1,1), to provide a 7-bit signal RR₂.

A SBox92 (S92) Operator 350 generates a 9-bit signal y0, y1, . . . , y8 from the signal RL₂ by Eq. (1). A ZE2 unit 360 receives the signal RR₂, adds two zeroes to the MSB of the signal RR₂, and outputs a 9-bit signal. An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the S92 operator 350 and the ZE2 unit 360 to provide a 9-bit signal RL₃. A TR2 unit 370 removes two zero bits from the MSBs of the 9-bit signal RL₃. A SBox72 (S72) operator 380 generates a 7-bit signal y0, y1, . . . , y6 from the input signal RR₂ (=RR₃) using Eq. (2). Another Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the TR2 370 and the S72 operator 380 to provide a 7-bit signal RR₄.

The 9-bit signal RL₃ (=RL₄) and the 7-bit signal RR₄ are operated, resulting in a 16-bit ciphertext RL₄//RR₄.

As described above, the S91 operator 310 and the S92 operator 350 each sequentially perform an AND operation to perform a logical “AND” and an exclusive-OR operation to perform a logical “Exclusive-OR” using Eq. (1), to thereby generate an output signal y0, y1, . . . , y8. Similarly, the S71 operator 340 and the S72 operator 380 sequentially perform an AND operation to perform a logical “AND” and an exclusive-OR operation to perform a logical “Exclusive-OR” using Eq. (2), to thereby generate an output signal y0, y1, . . . , y6. Consequently, the encryption speed is decreased. Moreover, a gate delay involved in the operations of the S91, S92, S71 and S72 operators 310, 350, 340, and 360 gradually increases glitch.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an encryption method for generating a ciphertext bit stream of length 2 n from a plaintext bit stream of length 2 n.

It is another object of the present invention to provide an encryption apparatus for generating a ciphertext bit stream of length 2 n from a plaintext bit stream of length 2 n.

To achieve the above objects, in an encryption method for dividing a first plaintext bit stream of length 2 n into first and second sub-bit streams of length n, dividing a second plaintext bit stream of length 2 n into third and fourth sub-bit streams of length n, and generating a ciphertext bit stream of length 2 n from the first, second, third and fourth sub-bit streams using 2-rounds of encryption, first and second ciphertext bit streams of length n are generated by encrypting the first and second sub-bit streams with predetermined first encryption codes KO_(1,1), KO_(1,2), KO_(1,3), KI_(1,1), KI_(1,2), and KI_(1,3), the second ciphertext bit stream being output with a predetermined time delay from the first ciphertext bit stream, in a first-round encryption. A first operated ciphertext bit stream is generated by performing a logical exclusive-OR-operation on the first ciphertext bit stream and the third sub-bit stream, and a second operated ciphertext bit stream is operated by performing a logical exclusive-OR-operation on the second ciphertext bit stream and the fourth sub-bit stream. In a second-round of encryption, third and fourth ciphertext bit streams of length n are generated by encrypting the first operated ciphertext bit stream and the second operated ciphertext bit stream with predetermined second encryption codes KO_(2,1), KO_(2,2), K_(2,3), KI_(2,1), KI_(2,2), and KI_(2,3) and the third and fourth ciphertext bit streams a concurrently output.

In an encryption apparatus for dividing a first plaintext bit stream of length 2 n into first and second sub-bit streams of length n, dividing a second plaintext bit stream of length 2 n into third and fourth sub-bit streams of length n, and generating a ciphertext bit stream of length 2 n from the first, second, third and fourth sub-bit streams using 2-rounds of encryption, a first ciphering unit receives the first and second sub-bit streams, and generates first and second ciphertext bit streams of length n by encrypting the first and second sub-bit streams with predetermined first encryption codes KO_(1,1), KO_(1,2), KO_(1,3), KI_(1,1), KI_(1,2), and KI_(1,3). Here, the second ciphertext bit stream is output with a predetermined time delay from the first ciphertext bit stream. An operating unit generates a first operated ciphertext bit stream by performing a logical exclusive-OR operation on the first ciphertext bit stream and the third sub-bit stream, and generates a second operated ciphertext bit stream by performing a logical exclusive-OR-operation on the second ciphertext bit stream with the fourth sub-bit stream. A second ciphering unit receives the first operated ciphertext bit stream and the second operated ciphertext bit stream having the predetermined time delay, generates third and fourth ciphertext bit streams of length n by encrypting the first operated ciphertext bit stream and the second operated ciphertext bit stream with predetermined second encryption codes KO_(2,1), KO_(2,2), KO_(2,3), KI_(2,1), KI_(2,2), and KI_(2,3) and concurrently outputs the third and fourth ciphertext bit streams.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block diagram illustrating an example of a conventional KASUMI algorithm;

FIG. 2A is a block diagram illustrating an example of FOi units illustrated in FIG. 1;

FIG. 2B is a block diagram illustrating another example of the FOi units illustrated in FIG. 1;

FIG. 3 is a block diagram illustrating an example of f1_(i, j) sub-ciphers illustrated in FIGS. 2A and 2B;

FIG. 4 is a block diagram illustrating an example of a KASUMI algorithm according to the present invention;

FIG. 5 is a block diagram illustrating an example of SLIMFOi units illustrated in FIG. 4 according to the present invention; and

FIG. 6 is a block diagram illustrating an example of f1_(i, j) sub-ciphers illustrated in FIG. 5 according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described herein below with reference to the accompanying drawings. Also, a description of known functions and configurations have been omitted for conciseness.

A KASUMI algorithm according to the present invention is a ciphering algorithm used as the f8 confidentiality and f9 integrity algorithms. The f8 confidentiality algorithm encrypts a plaintext signal having a predetermined number of bits by exclusive-OR-operating the plaintext with an encryption key and decrypts a ciphertext by exclusive-OR-operating the ciphertext with the encryption key. The f9 integrity algorithm derives a message authentication code from a received signal. The KASUMI algorithm, as previously discussed, has emerged as a significant issue to confidentiality and integrity.

FIG. 4 is a block diagram illustrating an example of a KASUMI algorithm according to the present invention. Referring to FIG. 4, a KASUMI of the present invention provides a 64-bit output ciphertext from a 64-bit input plaintext using first, second and third encryption keys. The 64-bit input signal is divided into a 32-bit signal L₀ and another 32-bit signal R₀. FLi units (1≦i≦8) 410 to 480 and SLIMFOi units (1≦i≦4) 510 to 540 are used to encrypt the signals L₀ and R₀ using corresponding encryption keys KO_(i) (1≦i≦8) and KI_(i) (1≦i≦8) to output a 64-bit ciphertext.

Describing FIG. 4 in more detail, an FL1 unit 410 encrypts the input 32-bit signal L₀ with an encryption key KL₁ and outputs a ciphertext L₁. An SLIMFO1 unit 510 encrypts the 32-bit ciphertext L₁ with encryption keys KO₁ and KI₁, outputs a signal SR₁ by encrypting the signal L₁ with the 32-bit signal R₀, and then outputs a signal R₁ by encrypting the signal SR₁ with encryption keys KO₂ and KI₂. An FL2 unit 420 encrypts the signal R₁ with an encryption key KL₂ and outputs a ciphertext R₂. An Exclusive-OR operation is performed to logically “exclusive OR” the signals L₀ and R₂ to provide a signal L₂ (=SL₁).

An FL3 unit 430 encrypts the signal L₂ with an encryption key KL₃ and outputs a ciphertext L₃. An SLIMFO2 unit 520 encrypts the signal L₃ with encryption keys KO₃ and KI₃, outputs a signal SR₂ by operating the encrypted signal L₃ with the signal SR₁, and then outputs a signal R₃ by encrypting the signal SR₂ with encryption keys KO₄ and KI₄. An FL4 unit 440 encrypts the signal R₃ with an encryption key KL₄ and outputs a ciphertext R₄. An Exclusive-OR operation is performed to logically “exclusive OR” the signals L₂ (=SL₁) and R₄ to provide a signal L₄ (=SL₂).

An FL5 unit 450 encrypts the signal L₄ with an encryption key KL₅ and outputs a ciphertext L₅. An SLIMFO3 unit 530 encrypts the signal L₅ with encryption keys KO₅ and KI₅, outputs a signal SR₃ by operating the encrypted signal L₃ with the signal SR₂, and then outputs a signal R₅ by encrypting the signal SR₃ with encryption keys KO₆ and KI₆. An FL6 unit 460 encrypts the signal R₅ with an encryption key KO₆ and outputs a ciphertext R₆. An Exclusive-OR operation is performed to logically “exclusive OR” the signals L₄ (=SL₂) and R₆ to provide a signal L₆ (=SL₃).

An FL7 unit 470 encrypts the signal L₆ with an encryption key KL₇ and outputs a ciphertext L₇. An SLIMFO4 unit 540 encrypts the signal L₇ with encryption keys KO₇ and KI₇, outputs a signal SR₄ by operating the encrypted signal L₇ with the signal SR₃, and then outputs a signal R₇ by encrypting the signal SR₄ with encryption keys KO₈ and KI₈. An FL8 unit 480 encrypts the signal R₇ with an encryption key KL₈ and outputs a ciphertext R₈. The signals L₆ (=SL₃) and R₈ are exclusive-OR-operated, resulting in a signal L₈ (=SL₄). Consequently, the eight FLi units (1≦i≦8) 410 to 480 and the four SLIMFOi units (1≦i≦4) 510 to 540 encrypt the 64-bit plaintext and output the 64-bit ciphertext, that is, the 32-bit signal SL₄//the 32-bit SR₄.

FIG. 5 is a block diagram illustrating an example of the SLIMFOi units illustrated in FIG. 4 according to an embodiment of the invention. Referring to FIG. 5, a SLIMFOi unit is an ith SLIMFO unit and implemented using parallel operations of signals in two FOi units. The SLIMFO1 unit 510 of FIG. 4 will be described by way of example. The SLIMFO1 unit 510 comprises an FO1 cipher 501 and an FO2 cipher 502. Each FO cipher includes f1_(i,j) sub-ciphers (1≦i≦2, 1≦j≦3), for 3-round encryption.

The signal resulting from encrypting the 32-bit signal L₀ with the encryption key KL₁ in FIG. 4 is divide into a 16-bit signal L₀ (=L₁) and a 16-bit signal R₀ (=R₁) in the FO1 cipher 501. A signal L₂ is generated by performing a logical exclusive-OR operation on the signal L₁ with a sub-encryption key KO_(1,1). An f1_(1,1) sub-cipher 511 generates a signal L_(2D) by encrypting the signal L₂ with a sub-encryption key KI_(1,1). A delay (D6) 600 delays the signal R₁ and outputs a delayed signal R_(1D). A signal L₃ is generated by performing a logical exclusive-OR operation on the signals R_(1D) and L_(2D). Meanwhile, a signal R₂ is generated by performing a logical exclusive-OR operation on the signal R₁ with a sub-encryption key KO_(1,2). An FL_(1,2) sub-cipher 512 generates a signal R_(2D) by encrypting the signal R₂ with a sub-encryption key KI_(1,2). A signal R₃ is generated by performing a logical exclusive-OR operation on the signals R_(2D) and L₃. A signal L₄ is generated by performing a logical exclusive-OR operation on the signal L₃ with a sub-encryption key KO_(1,3). An f1_(1,3) sub-cipher 513 generates a signal L_(4D) by encrypting the signal L₄ with a sub-encryption key KI_(1,3). A delay (D7) 620 delays the signal R₃ and outputs a delayed signal R_(3D). A 16-bit signal L₅ is generated by performing a logical exclusive-OR operation on the signals R_(3D) and L_(4D).

The 32-bit signal R₀ which was divided from the 64-bit signal in FIG. 4 is further divided into a 16-bit signal L_(0′) and a 16-bit signal R_(0′) in the FO2 cipher 502. A signal L₆ is generated by performing a logical exclusive-OR operation on the signal L_(0′) using the 16-bit signal L₅. Meanwhile, a signal R₄ is generated by performing a logical exclusive-OR operation on the signal R_(0′) using the 16-bit signal R₃. A signal R₅ is generated by performing a logical exclusive-OR operation on the signal R₄ using a sub-encryption key KO_(2,1). An f1_(2,1) sub-cipher 514 generates a signal R_(5D) by encrypting the signal R₅ with a sub-encryption key KI_(2,1). A signal R₆ is generated by performing a logical exclusive-OR operation on the signals R_(5D) and L₆. That is, the f1_(1,3) sub-cipher 513 and the f1_(2,1) sub-cipher 514 synchronize the signal L₆ to the signal R₆ without using delays. A signal L₇ is generated by performing a logical exclusive-OR operation on the signal L₆ with a 16-bit sub-encryption key KO_(2,2). An f1_(2,2) sub-cipher 515 generates a signal L_(7D) by encrypting the signal L₇ with a 16-bit sub-encryption key KI_(2,2). A delay (D8) 640 delays the signal R₆ and outputs a delayed signal R_(6D). A signal L₈ is generated by performing a logical exclusive-OR operation on the signals L_(7D) and R_(6D). A signal R₇ is generated by performing a logical exclusive-OR operation on the signal R₆ with a 16-bit sub-encryption key KO_(2,3). An f1_(2,3) sub-cipher 516 generates a signal R_(7D) by encrypting the signal R₇ with a 16-bit sub-encryption key KI_(2,3). A signal R₈ is generated by performing a logical exclusive-OR operation on the signals R_(7D) and L₈. Consequently, a 32-bit ciphertext L₈∥R₈ is generated by operating the 16-bit signal L₈ with the 16-bit signal R₈.

As described above, the SLIMFO1 unit encrypts the input plaintext by processing the 16-bit signals L₀ and R₀ in parallel in the FO1 cipher 501 and processing the 16-bit signals L_(0′) and R_(0′) in parallel in the FO2 cipher 502. The parallel processing of the 32-bit signals L₀ and R₀ which were divided from the 64-bit input signal in the SLIMFOi units remarkably increases encryption speed and reduces the number of delays used to synchronize a delayed signal to a non-delayed signal.

FIG. 6 is a block diagram illustrating an example of the f1_(i,j) sub-ciphers illustrated in FIG. 5 according to an embodiment of the invention. By way of example, the f1_(1,1) sub-cipher 511 will be described below.

Referring to FIG. 6, the f1_(1,1) sub-cipher 511 includes a first ciphering unit and a second ciphering unit. In the first ciphering unit, a 16-bit input signal is divided into a 9-bit signal RL₀ and a 7-bit signal RR₀. An S91 operator 710 generates a 9-bit signal y0, y1, . . . , y8 from the input signal RL₀ by y0=(x0x2)⊕x3⊕(x2x5)⊕(x5x6)⊕(x0x7)⊕(x1x7)⊕(x2x7)⊕(x4x8)⊕(x5x3)⊕(x7x8)⊕‘1’; y1=x1⊕(x0x1)⊕(x2x3)⊕(x0x4)⊕(x1x4)⊕(x0x5)⊕(x3x5)⊕x6⊕(x1x7)⊕(x2x7)⊕(x5x8)⊕‘1’; y2=x1⊕(x0x3)⊕(x3x4)⊕(x0x5)⊕(x2x6)⊕(x3x6)⊕(x5x6)⊕(x4x7)⊕(x5x7)⊕(x6x7)⊕x8⊕(x0x8)⊕‘1’; y3=x0⊕(x1x2)⊕(x0x3)⊕(x2x4)⊕x5⊕(x0x6)⊕(x1x6)⊕(x4x7)⊕(x0x8)⊕(x1x8)⊕(x7x8); y4=(x0x1)⊕(x1x3)⊕x4⊕(x0x5)⊕(x3x6)⊕(x0x7)⊕(x6x7)⊕(x1x8)⊕(x2x3)⊕(x3x8); y5=x2⊕(x1x4)⊕(x4x5)⊕(x0x6)⊕(x1x6)⊕(x3x7)⊕(x4x7)⊕(x6x7)⊕(x5x3)⊕(x6x8)⊕(x7x8)⊕‘1’; y6=x0⊕(x2x3)⊕(x1x5)⊕(x2x5)⊕(x4x5)⊕(x3x6)⊕(x4x6)⊕(x5x6)⊕x7⊕(x1x8)⊕(x3x8)⊕(x5x8)⊕(x7x8); y7=(x0x1)⊕(x0x2)⊕(x1x2)⊕x3⊕(x0x3)⊕(x2x3)⊕(x4x5)⊕(x2x6)⊕(x3x5)⊕(x2x7)⊕(x5x7)⊕x8⊕‘1’; y8=(x0x1)⊕x2⊕(x1x2)⊕(x3x4)⊕(x1x5)⊕(x2x5)⊕(x1x6)⊕(x4x6)⊕x7⊕(x2x8)⊕(x3x8);  (3)

That is, the S91 operator 710 generates the 9-bit signal y0, y2, . . . , y8 by performing parallel logical AND operations and then performing a logical exclusive-OR operation of a 9-bit signal x0, x1, . . . , x8 in parallel. A ZE1 unit 720 receives the signal RR₀, adds two zeroes to the MSB of the signal RR₀, and outputs a 9-bit signal. An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the S91 operator 710 and the ZE1 unit 720 to provide a 9-bit signal RL₁. Another Exclusive-OR operation is performed to logically “exclusive OR” the signal RL₁ and a 9-bit sub-encryption key KI_(1,1,2), to provide a 9-bit signal RL₂. The signal RL₂ is temporarily stored in a first register (register 1) 800.

Simultaneously, an S71 operator 740 generates a 7-bit signal y0, y1, . . . , y6 from the input signal RR₀ (=RR₁) by y0=(x1x3)⊕x4⊕(x0x1x4)⊕x5⊕(x2x5)⊕(x3x4x5)⊕x6⊕(x0x6)⊕(x1x6)⊕(x3x6)⊕(x2x4x6)⊕(x1x5x6)⊕(x4x5x6); y1=(x0x1)⊕(x1x4)⊕(x2x4)⊕x5⊕(x1x2x5)⊕(x0x3x5)⊕x6⊕(x0x2x3)⊕(x3x6)⊕(x4x5x6)⊕‘1’; y2=x0⊕(x0x3)⊕(x2x3)⊕(x1x2x4)⊕(x0x3x4)⊕(x1x5)⊕(x0x2x5)⊕(x0x5)⊕(x0x4x6)⊕(x2x6)⊕(x4x6)⊕‘1’; y3=x1⊕(x0x1x2)⊕(x1x4)⊕(x3x4)⊕(x0x5)⊕(x0x1x5)⊕(x2x3x5)⊕(x1x4x5)⊕(x2x6)⊕(x1x3x6); y4=(x0x2)⊕x3⊕(x1x3)⊕(x1x4)⊕(x0x1x4)⊕(x2x3x4)⊕(x0x5)⊕(x1x3x5)⊕(x0x4x5)⊕(x1x6)⊕(x3x6)⊕(x0x3x6)⊕(x5x6)⊕‘1’; y5=x2⊕(x0x2)⊕(x0x3)⊕(x1x2x3)⊕(x0x1x4)⊕(x0x5)⊕(x2x5)⊕(x4x5)⊕(x1x6)⊕(x1x2x6)⊕(x0x3x6)⊕(x3x4x6)⊕(x2x5x6)⊕‘1’; y6=(x1x2)⊕(x1x1x3)⊕(x0x4)⊕(x1x5)⊕(x3x5)⊕x6⊕(x0x1x6)⊕(x2x3x6)⊕(x1x4x5)⊕(x0x5x6);  (4)

That is, the S71 operator 740 generates the 7-bit signal y0, y2, . . . , y6 by performing parallel logical AND operations and then performing a logical exclusive-OR operation of a 7-bit signal x0, x1, . . . , x6 in parallel. A TR1 unit 730 removes two zeroes from the MSBs of the 9-bit signal RL₁ and outputs the resulting 7-bit signal. A 7-bit signal RR₂ is generated by performing a logical exclusive-OR operation on the outputs of the TR1 730 and the S71 operator 740 with a sub-encryption key KI_(1,1,1,). The signal RR₂ is temporarily stored in the first register 800. Upon receipt of a first clock signal CLK1 from a controller (not shown), the register 800 simultaneously outputs the 9-bit signal RL₂ and the 7-bit signal RR₂. Thus the register 800 functions to synchronize the output timings of signals according to delay involved with encryption in the S91 operator 710, the ZE1 unit 720, the TR1 unit 730, and the S71 operator 740.

In the second ciphering unit, an S92 operator 750 generates a 9-bit signal y0, y1, . . . , y8 from the 9-bit signal RL₂ received from the register 800 using Eq. (3). A ZE2 unit 760 adds two zeroes to the MSB of the signal RR₂ received from the register 800 and outputs a 9-bit signal. An Exclusive-OR operation is performed to logically “exclusive OR” the outputs of the S92 operator 750 and the ZE2 unit 760 to provide a 9-bit signal RL₃. The signal RL₃ is temporarily stored in a second register (register 2) 820.

Simultaneously, an S72 operator 780 generates a 7-bit signal y0, y1, . . . , y6 from the 7-bit signal RR₂ (=RR₃) using Eq. (4). A TR2 unit 770 removes two zeroes from the MSBs of the 9-bit signal RL₃ and outputs the resulting 7-bit signal. A 7-bit signal RR₄ is generated by performing a logical exclusive-OR-operation on the outputs of the TR₂ 770 and the S72 operator 780. The signal RR₄ is temporarily stored in the second register 820.

Upon receipt of a second clock signal CLK2 from the controller; the register 820 simultaneously outputs the 9-bit signal RL₄ and the 7-bit signal RR₄. Thus the register 820 functions to synchronize the output timings of signals according to the delay involved with the encryption in the S92 operator 750, the ZE2 unit 760, the TR2 unit 770, and the S72 operator 780.

As described above, the S91 operator 710 and the S92 operator 750 each output a 9-bit signal y0, y1, . . . , y8 by performing parallel logical AND operations and then performing a logical exclusive-OR operation according to Eq. (3). The S71 operator 740 and the S72 operator 780 each output a 7-bit signal y0, y1, . . . , y6 by parallel AND operations and then exclusive-OR operation according to Eq. (4). Therefore, encryption speed is remarkably increased. Furthermore, the use of the registers 800 and 820 for signal timing synchronization enables output of an accurate ciphertext.

In accordance with the present invention, (1) parallel computation of input signals increases signal processing speed; (2) due to synchronization of the output timings of a delayed signal and a non-delayed signal, an accurate ciphertext is achieved and thus an encryption system is further stabilized; and (3) the decrease in devices used for synchronization reduces required chip capacity and production cost.

While the invention has been shown and described with reference to a certain embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 

1. An encryption method for dividing a first plaintext bit stream of length 2 n into first and second sub-bit streams of length n, dividing a second plaintext bit stream of length 2 n into third and fourth sub-bit streams of length n, and generating a ciphertext bit stream of length 2 n from the first, second, third and fourth sub-bit streams using 2-rounds of encryption, the method comprising the steps of: performing the first-round of encryption by encrypting the received the first and second sub-bit streams with predetermined first encryption codes an odd number of times and outputting first and second ciphertext bit streams, wherein the first ciphertext bit stream is encrypted a first number of times and the second ciphertext bit stream is encrypted a second number of times, the second number of times exceeding the first number of times; generating a first operated ciphertext bit stream by performing a logical exclusive-OR-operation on the first ciphertext bit stream and the third sub-bit stream; generating a second operated ciphertext bit stream by performing a logical exclusive-OR operation on the second ciphertext bit stream and the fourth sub-bit stream; and performing the second-round of encryption by encrypting the received first operated ciphertext bit stream and the second operated ciphertext bit stream with predetermined second encryption codes an odd number of times and concurrently outputting third and fourth ciphertext bit streams of length n, wherein the third ciphertext bit stream is encrypted a third number of times and the fourth ciphertext bit stream is encrypted the fourth number of times, the fourth number of times exceeding the third number of times, wherein the last encryption for the second ciphertext bit stream in the first-round is performed at the same time of performing the first encryption of the first operated ciphertext bit stream in the second round of encryption.
 2. The encryption apparatus of claim 1, wherein the predetermined first encryption codes comprises at least one of KO_(1,1), KO_(1,2), KO_(1,3), KI_(1,1), KI_(1,2), and KI_(1,3).
 3. The encryption apparatus of claim 1, wherein the predetermined second encryption codes comprises at least one of KO_(2,1), KO_(2,2), KO_(2,3), KI_(2,1), KI_(2,2), and KI_(2,3).
 4. The encryption method of claim 2, wherein the first-round encryption step comprises the steps of: generating a first signal by performing a logical exclusive-OR operation on the first sub-bit stream and the first encryption code KO_(1,1) to provide a first exclusive-OR operated bitstream, encrypting the first exclusive-OR-operated bit stream with the first encryption code KI_(1,1) to provide a first encrypted signal, and performing a logical exclusive-OR operation on the first encrypted signal and the second sub-bit stream delayed by time required for the encryption; generating the first operated ciphertext bit stream by performing a logical exclusive-OR-operation on the second sub-bit stream and the first encryption code KO_(1,2), to provide a second exclusive-OR operated bitstream encrypting the second exclusive-OR-operated bit stream with the first encryption code KI_(1,2) to provide a second encrypted signal, and performing a logical exclusive-OR-operation on the second encrypted signal and the first signal; generating the second operated ciphertext bit stream by performing a logical exclusive-OR-operation on the first signal and the first encryption code KO_(1,3) to provide a third exclusive-OR operated bitstream, encrypting the third exclusive-OR-operated bit stream with the first encryption code KI_(1,3), and performing a logical exclusive-OR-operation on the encrypted signal with the first sub-bit stream delayed by time required for the encryption.
 5. The encryption method of claim 3, wherein the second-round encryption step comprises the steps of: generating a second signal by performing a logical exclusive-OR-operation on the first operated ciphertext bit stream and the second encryption code KO_(2,1) to provide a fourth exclusive-OR operated bitstream, encrypting the fourth exclusive-OR-operated bit stream with the second encryption code KI_(2,1) to provide a third encrypted signal, performing a logical exclusive-OR-operation on the third encrypted signal and the second operated ciphertext bit stream to provide a fifth exclusive-OR operated bitstream; generating the third operated ciphertext bit stream by performing a logical exclusive-OR-operation on the second operated ciphertext bit stream and the second encryption code KO_(2,2), encrypting the fifth exclusive-OR-operated bit stream with the second encryption code KI_(2,2) to provide a fourth encrypted signal, and performing a logical exclusive-OR-operation on the fifth encrypted signal and the second signal delayed by time required for the encryption; and generating the fourth ciphertext bit stream by performing a logical exclusive-OR-operation on the second signal and the second encryption code KO_(2,3) encrypting the sixth exclusive-OR-operated bit stream with the second encryption code KI_(2,3), and performing a logical exclusive-OR-operation on the encrypted signal with the third operated ciphertext bit stream.
 6. The encryption method of claim 5, wherein each of the encryptions includes first and second sub-encryptions, and outputs from the first and second sub-encryptions are stored and simultaneously retrieved according to an external clock signal.
 7. The encryption method of claim 5, wherein a 16-bit input bit stream is divided into a 9-bit stream and a 7-bit stream, a 9-bit ciphertext bit stream is generated from the 9-bit stream using a first equation, and a 7-bit ciphertext bit stream is generated from the 7-bit stream using a second equation in each of the sub-encryptions, wherein said first equation comprises y0=(x0x2)⊕x3⊕(x2x5)⊕(x5x6)⊕(x0x7)⊕(x1x7)⊕(x2x7)⊕(x4x8)⊕(x5x3)⊕(x7x8)⊕‘1’; y1=x1⊕(x0x1)⊕(x2x3)⊕(x0x4)⊕(x1x4)⊕(x0x5)⊕(x3x5)⊕x6⊕(x1x7)⊕(x2x7)⊕(x5x8)⊕‘1’; y2=x1⊕(x0x3)⊕(x3x4)⊕(x0x5)⊕(x2x6)⊕(x3x6)⊕(x5x6)⊕(x4x7)⊕(x5x7)⊕(x6x7)⊕x8⊕(x0x8)⊕‘1’; y3=x0⊕(x1x2)⊕(x0x3)⊕(x2x4)⊕x5⊕(x0x6)⊕(x1x6)⊕(x4x7)⊕(x0x8)⊕(x1x8)⊕(x7x8); y4=(x0x1)⊕(x1x3)⊕x4⊕(x0x5)⊕(x3x6)⊕(x0x7)⊕(x6x7)⊕(x1x0)⊕(x2x3)⊕(x3x0); y5=x2⊕(x1x4)⊕(x4x5)⊕(x0x6)⊕(x0x6)⊕(x3x7)⊕(x4x7)⊕(x6x7)⊕(x5x3)⊕(x6x8)⊕(x7x8)⊕‘1’; y6=x0⊕(x2x3)⊕(x1x5)⊕(x2x5)⊕(x4x5)⊕(x3x6)⊕(x4x6)⊕(x5x6)⊕x7⊕(x1x8)⊕(x3x8)⊕(x5x8)⊕(x7x8); y7=(x0x1)⊕(x0x2)⊕(x1x2)⊕x3⊕(x0x3)⊕(x2x3)⊕(x4x5)⊕(x2x6)⊕(x3x5)⊕(x2x7)⊕(x5x7)⊕(x8⊕‘1’; y8=(x0x1)⊕x2⊕(x1x2)⊕(x3x4)⊕(x1x5)⊕(x2x5)⊕(x1x6)⊕(x4x6)⊕x7⊕(x2x8)⊕(x3x8); and said second equation comprises y0=(x1x3)⊕x4⊕(x0x1x1)⊕x5⊕(x2x5)⊕(x3x4x5)⊕x6⊕(x0x6)⊕(x1x6)⊕(x3x6)⊕(x2x4x6)⊕(x1x5x6)⊕(x4x5x6); y1=(x0x1)⊕(x1x4)⊕(x2x4)⊕x5⊕(x1x2x5)⊕(x0x3x5)⊕x5⊕(x0x2x3)⊕(x3x6)⊕(x4x5x6)‘1’; y2=x0⊕(x0x3)⊕(x2x3)⊕(x1x2x4)⊕(x0x3x4)⊕(x1x5)⊕(x0x2x5)⊕(x0x5)⊕(x0x1x6)⊕(x2x6)⊕(x1x6)⊕‘1’; y3=x1⊕(x0x1x2)⊕(x1x4)⊕(x3x4)⊕(x0x5)⊕(x0x1x5)⊕(x2x3x5)⊕(x1x4x5)⊕(x2x6)⊕(x1x3x6); y4=(x0x2)⊕x3⊕(x1x3)⊕(x1x4)⊕(x0x1x4)⊕(x2x3x4)⊕(x0x5)⊕(x1x3x5)⊕(x0x4x5)⊕(x1x6)⊕(x3x6)⊕(x0x3x6)⊕(x5x6)⊕‘1’; y5=x2⊕(x0x2)⊕(x0x3)⊕(x1x2x3)⊕(x0x1x4)⊕(x0x5)⊕(x2x5)⊕(x4x5)⊕(x1x6)⊕(x1x2x6)⊕(x0x3x6)⊕(x3x4x6)⊕(x2x5x6)⊕‘1’; y6=(x1x2)⊕(x1x1x3)⊕(x0x4)⊕(x1x5)⊕(x3x5)⊕x6⊕(x0x1x6)⊕(x2x3x6)⊕(x1x4x5)⊕(x0x5x6);
 8. An encryption apparatus for dividing a first plaintext bit stream of length 2 n into first and second sub-bit streams of length n, dividing a second plaintext bit stream of length 2 n into third and fourth sub-bit streams of length n, and generating a ciphertext bit stream of length 2 n from the first, second, third and fourth sub-bit streams using 2-rounds of encryption, the apparatus comprising: a first ciphering unit for receiving the first and second sub-bit streams, and generating first and second ciphertext bit streams of length n by encrypting the first and second sub-bit streams with predetermined first encryption codes KO_(1,1), KO_(1,2), KO_(1,3), KI_(1,1), KI_(1,2), and KI_(1,3) an odd number of times, wherein the first ciphertext bit stream is encrypted a first number of times and the second ciphertext bit stream is encrypted a second number of times, the second number of times exceeding the first number of times; an operating unit for generating a first operated ciphertext bit stream by performing a logical exclusive-OR-operation on the first ciphertext bit stream and the third sub-bit stream and generating a second operated ciphertext bit stream by performing a logical exclusive-OR-operation on the second ciphertext bit stream with the fourth sub-bit stream; and a second ciphering unit for receiving the first operated ciphertext bit stream and the second operated ciphertext bit stream comprising the predetermined time delay, generating third and fourth ciphertext bit streams of length n by encrypting the first operated ciphertext bit stream and the second operated ciphertext bit stream with predetermined second encryption codes KO_(2,1), KO_(2,2), KO_(2,3), KI_(2,1), KI_(2,2), and KI_(2,3) an odd number of times, and concurrently outputting the third and fourth ciphertext bit streams, wherein the third ciphertext bit stream is encrypted a third number of times and the fourth ciphertext bit stream is encrypted the fourth number of times, the fourth number of times exceeding the third number of times, wherein the last encryption for the second ciphertext bit stream in the first-round of encryption is performed at the same time of performing the first encryption for the first operated ciphertext bit stream in the second-round of encryption.
 9. The encryption apparatus of claim 8, wherein the first ciphering unit comprises: a first block comprising a first exclusive-OR operator for performing a logical exclusive-OR operation on the first sub-bit stream and the first encryption code KO_(1,1), a first sub-cipher for encrypting the exclusive-OR-operated bit stream with the first encryption code KI_(1,1), and a second exclusive-OR operator for generating a first signal by performing a logical exclusive-OR operation on the encrypted signal with the second sub-bit stream being delayed to provide time for the encryption; a second block comprising a third exclusive-OR operator for performing a logical exclusive-OR operation on the second sub-bit stream and the first encryption code KO_(1,2), a second sub-cipher for encrypting the exclusive-OR-operated bit stream with the first encryption code KI_(1,2), and a fourth exclusive-OR operator for generating the first operated ciphertext bit stream by performing a logical exclusive-OR operation on the encrypted signal and the first signal; and a third block comprising a fifth exclusive-OR operator for performing a logical exclusive-OR operation on the first signal and the first encryption code KO_(1,3), a third sub-cipher for encrypting the exclusive-OR-operated bit stream with the first encryption code KI_(1,3), and a sixth exclusive-OR operator for generating the second operated ciphertext bit stream by performing a logical exclusive-OR-operation on the encrypted signal and the first sub-bit stream delayed by time required for the encryption.
 10. The encryption apparatus of claim 8, wherein the second ciphering unit comprises: a fourth block comprising a seventh exclusive-OR operator for exclusive-OR-operating the first operated ciphertext bit stream with the second encryption code KO_(2,1), a fourth sub-cipher for encrypting the exclusive-OR-operated bit stream with the second encryption code KI_(2,1), and an eighth exclusive-OR operator for generating a second signal by performing a logical exclusive-OR-operation on the encrypted signal and the second operated ciphertext bit stream; a fifth block comprising a ninth exclusive-OR operator for exclusive-OR-operating the second operated ciphertext bit stream with the second encryption code KO_(2,2), a fifth sub-cipher for encrypting the exclusive-OR-operated bit stream with the second encryption code KI_(2,2), and a tenth exclusive-OR operator for generating the third ciphertext bit stream by performing a logical exclusive-OR-operation on the encrypted signal and the second signal delayed by time required for the encryption; and a sixth block comprising an eleventh exclusive-OR operator for performing a logical exclusive-OR operation on the second signal with the second encryption code KO_(2,3), a sixth sub-cipher for encrypting the exclusive-OR-operated bit stream with the second encryption code KI_(2,3), and a twelfth exclusive-OR operator for generating the fourth ciphertext bit stream by performing a logical exclusive-OR operation on the encrypted signal and the third ciphertext bit stream.
 11. The encryption apparatus of claim 10, wherein each of the first to sixth sub-ciphers includes first and second sub-ciphering units, and a register for storing the outputs of the first and second sub-ciphering units and simultaneously retrieving the outputs according to an external clock signal.
 12. The encryption apparatus of claim 11, wherein each of the first and second sub-ciphering units divides a 16-bit input bit stream into a 9-bit stream and a 7-bit stream, and generates a 9-bit ciphertext bit stream from the 9-bit stream using a third equation, and a 7-bit ciphertext bit stream from the 7-bit stream using a fourth equation, said third equation comprising y0=(x0x2)⊕x3⊕(x2x5)⊕(x5x6)⊕(x0x7)⊕(x1x7)⊕(x2x7)⊕(x4x8)⊕(x5x3)⊕(x7x8)⊕‘1’; y1=x1⊕(x0x1)⊕(x2x3)⊕(x0x4)⊕(x1x4)⊕(x0x5)⊕(x3x5)⊕x6⊕(x1x7)⊕(x2x7)⊕(x5x8)⊕‘1’; y2=x1⊕(x0x3)⊕(x3x4)⊕(x0x5)⊕(x2x6)⊕(x3x6)⊕(x5x6)⊕(x4x7)⊕(x5x7)⊕(x6x7)⊕x8⊕(x0x8)⊕‘1’; y3=x0⊕(x1x2)⊕(x0x3)⊕(x2x4)⊕x5⊕(x0x6)⊕(x1x6)⊕(x4x7)⊕(x0x8)⊕(x1x8)⊕(x7x8); y4=(x0x1)⊕(x1x3)⊕x4⊕(x0x5)⊕(x3x6)⊕(x0x7)⊕(x6x7)⊕(x1x8)⊕(x2x3)⊕(x3x8); y5=x2⊕(x1x4)⊕(x4x5)⊕(x0x6)⊕(x1x6)⊕(x3x7)⊕(x4x7)⊕(x6x7)⊕(x5x3)⊕(x6x8)⊕(x7x8)⊕‘1’; y6=x0⊕(x2x3)⊕(x1x5)⊕(x2x5)⊕(x4x5)⊕(x3x6)⊕(x4x6)⊕(x5x6)⊕x7⊕(x1x8)⊕(x3x8)⊕(x5x8)⊕(x7x8); y7=(x0x1)⊕(x0x2)⊕(x1x2)⊕x3⊕(x0x3)⊕(x2x3)⊕(x4x5)⊕(x2x6)⊕(x3x5)⊕(x2x7)⊕(x5x7)⊕x8⊕‘1’; y8=(x0x1)⊕x2⊕(x1x2)⊕(x3x4)⊕(x1x5)⊕(x2x5)⊕(x1x6)⊕(x4x6)⊕x7⊕(x2x8)⊕(x3x8); and said fourth equation comprising y0=(x1x3)⊕x4⊕(x0x1x4)⊕x5⊕(x2x5)⊕(x3x4x5)⊕x6⊕(x0x6)⊕(x1x6)⊕(x3x6)⊕(x2x4x6)⊕(x1x5x6)⊕(x4x5x6); y1=(x0x1)⊕(x1x4)⊕(x2x4)⊕x5⊕(x1x2x5)⊕(x0x3x5)⊕x6⊕(x0x2x3)⊕(x3x6)⊕(x4x5x6)⊕‘1’; y2=x0⊕(x0x3)⊕(x2x3)⊕(x1x2x4)⊕(x0x3x4)⊕(x1x5)⊕(x0x2x5)⊕(x0x5)⊕(x0x4x6)⊕(x2x6)⊕(x4x6)⊕‘1’; y3=x1⊕(x0x1x2)⊕(x1x4)⊕(x3x4)⊕(x0x5)⊕(x0x1x5)⊕(x2x3x5)⊕(x1x4x5)⊕(x2x6)⊕(x1x3x6); y4=(x0x2)⊕x3⊕(x1x3)⊕(x1x4)⊕(x0x1x4)⊕(x2x3x4)⊕(x0x5)⊕(x1x3x5)⊕(x0x4x5)⊕(x1x6)⊕(x3x6)⊕(x0x3x6)⊕(x5x6)⊕‘1’; y5=x2⊕(x0x2)⊕(x0x3)⊕(x1x2x3)⊕(x0x1x4)⊕(x0x5)⊕(x2x5)⊕(x4x5)⊕(x1x6)⊕(x1x2x6)⊕(x0x3x6)⊕(x3x4x6)⊕(x2x5x6)⊕‘1’; y6=(x1x2)⊕(x1x1x3)⊕(x0x4)⊕(x1x5)⊕(x3x5)⊕x6⊕(x0x1x6)⊕(x2x3x6)⊕(x1x4x5)⊕(x0x5x6); 